1. Field of the Invention
The present invention relates to a lubricating system for an engine, which comprises an oil pan for storing a lubricating oil therein, an oil cooler for cooling the lubricating oil, and an oil pump including first and second pump rotors contained in a casing. The oil pump is provided with a suction port commonly used for both the pump rotors, a first discharge port exclusively used for the first pump rotor and a second discharge port exclusively used for the second pump rotor. The suction port is in communication with the subsurface of the oil in the oil pan, the first discharge port being with an inlet port of the oil cooler, and the second discharge port being with the to-be-lubricated portions of an engine.
2. Description of the Prior Art
In FIG. 2 there is shown such a conventional lubricating system, wherein a lubricating oil cooled by an oil cooler 2 and a lubricating oil returned from a to-be-lubricated portion 3 of an engine are stored in an oil pan 1. An oil pump 4 comprises a first pump rotor serving for sucking the lubricating oil within the oil pan 1 into the pump through a common suction port 4a to pump it into a first discharge port 4e connected to an inlet port of the oil cooler, and a second pump rotor 4c serving for sucking the lubricating oil within the oil pan 1 into the pump through the common suction port 4a to supply it to the to-be-lubricated portion of the engine. An outlet port of the oil cooler is in communication with the oil pan.
The entire amount of the lubricating oil cooled in the oil cooler 2 is returned directly to the oil pan 1 to cool the lubricating oil stored in the oil pan 1, while the lubricating oil supplied from the oil pan to the to-be-lubricated portion 3 of the engine under the action of the second pump rotor 4c is returned to the oil pan 1 after lubrication. When the lubricating oil cooled in the oil cooler 2 has been returned directly to the oil pan in this manner, the temperature thereof is increased by the lubricating oil already stored in the oil pan 1. This causes the lubricating oil which has been satisfactorily not cooled to be resupplied to the to-be-lubricated portion, resulting in a failure to satisfactorily make efficient use of the function of the oil cooler.
In addition, because the lubricating oil is pressurized by the first pump rotor 4b, it enters the oil pan 1 with a substantial force. This causes the lubricating oil within the oil pan 1 to be vigorously stirred to produce bubbles in the oil. Before the bubbles disappear, the lubricating oil is pumped through the suction port 4a again to the oil cooler 2 and the to-be-lubricated portion 3 by the oil pump 4. If the lubricating oil with the bubbles entrained therein is supplied to the to-be-lubricated portion 3 of the engine, a failure in lubrication may be caused, resulting in a hindrance to the durability of the engine.